Due to the fact that electronic systems are developing towards lightness and shortness, high heat resistance, multi-functionality, high density, high reliability and low cost, powerful and high-speed signal transmission is thus a needed function. In the high-frequency field, the wireless infrastructure needs to provide enough low insertion loss, lower dielectric constant and less loss, to effectively improve the energy utilization rate. With the upcoming 5G age and application requirements of USB3.1, radio frequency products need to provide a wider bandwidth and needs to be downwards compatible with the 3G service and the 4G service. The common material of the cover film on the current market is the black polyimide film. As to the thinning design, the bottleneck of the black polyimide film is that it is extremely difficult and poorly operational to create a black polyimide film having the thickness less than 5 microns. Additionally, the adhesive like epoxy resin is poor in ion purity, poor in electrical property and high in cost. Hence, the above factors make the cover film difficult to adapt to the trend of current ultra-fine circuits and high-frequency high-speed transmission. As to the other ink type colored high-frequency cover film, a yellow ultrathin polyimide film coated with colored ink and having the thickness of 5-7.5 microns is used. This method also has the problems of being high in cost and large in thickness.
Further, the other manufacturing method currently used in the art is coating a yellow ultrathin polyimide film having the thickness of 5-7.5 microns with a black adhesive or ink, and attaching a bearing film to the black adhesive or the ink. The film belongs to a non-absorbent (bright) cover film, which causes high-glossiness (greater than 50 GU), poor processing operability, and high cost.
In the industrial design, to meet the requirements of the unexposed circuit design, beauty, visual protection and the like, a film with a low glossiness (low haze and high extinction ratio) is in demand. Black polyimide manufacturers in the market always add the powder to form surface scattering, so as to reduce the glossiness. For example, the Taiwanese patent application TW201132714A1 discloses a low-gloss black polyimide film doped with the ceramic powder. The Taiwanese patent application TW1481646B discloses a low-gloss polyimide film doped with polyimide powder. The South Korean patent application 10-2012-0036253 discloses a black polyimide film doped with carbon black. However, the method is not easy to be applied on an ultrathin polyimide film having the thickness of 5-7.5 microns.
In order to overcome the above defects, the colored polyimide cover is replaced by a high-frequency colored ultra-thin cover film, which is composed of a colored ink layer and a high-frequency adhesive. The advantages of the color ink layer, i.e., high color ink layer extinction, high flame retardancy, high hardness and the like, can be achieved by changing the resin, adding doping powder, changing the content of the powder, and designing a particle diameter.